20 PRINCIPLES OF AMERICAN FORESTRY. 



growth is often difficult to renew. As proof of the im- 

 portance of these conditions as affecting tree-growth we 

 have the commonly observed fact that the south and west 

 sides of steep hills and mountains are more likely to be 

 bare than any others. This can be very plainly seen on the 

 bluffs along the Mississippi River in Minnesota and Iowa. 

 A Western Slope receives the sun's rays obliquely, but in 

 the warmest part of the day gets the full force of the hot 

 dry southwest winds which are so common in many parts 

 of this country. The effect of such an exposure on growth 

 is about the same as the southern slope. On the mountain 

 ranges of the Pacific slopes of North America, the western 

 exposures receive the most rainfall and consequently have 

 the heaviest tree-growth. 



